Testing Heater Relay?

[Wayne 962]

A customer sent me this question, I have to run to a dentist appt right now, so I thought I would ask the forums to help him out:

Wayne,
My wife and I took my '85 911 out a couple of weeks ago for a short drive to do some errands. We both noticed that it gets HOT inside. I quickly deduced that my heater is stuck full on! I dug around my "office" at home and found a schematic of the heating system. I think that I knew about this last year but just had not time to get to it.
So here I am working through it. It appears that all of the wiring is present and in good shape and all components are present as well. I have come to conclusion that the point of failure with the heating system is either the relay in the engine compartment or the servo motor between the seats. A symptom that I have that should not be is that whenever I turn the key to ON or RUN the heater blower on top of the engine turns on. If I unplug the servo motor then it goes off.
Ultimately I have my bets that the relay just crapped out and the servo is fine. My question is this: Do you happen to know how to test this relay? The ones with 5 or 6 poles are easy, but this one has 12 and I can't seem to find any kind of internal schematic on it, so I do not exactly know its "input vs. output" behavior. I would like to know if at all possible before I chunk out the cash on a new relay; these are EXPENSIVE!

The second issue that I have is that I found that the blower fan under the dash does not work. I thought it did. Apparently the blower in the well in front of the fuel tank works fine but not the one under the dash. I guess I just never payed that much attention to it. I removed the 4-wire cable to this blower fan and checked for voltage.
Depending where the temperature handle (top one of the climate control
cluster) is set there is a voltage at the pins, so I believe that all of that is okay. When I plug in the harness the voltage at the + and
- inputs to the motor drop to zero. I suspect that the motor inside of the fan assembly has failed (although it looks to be in decent shape). I see that I can get a new fan/motor assembly (part #
911-571-320-32-M6) which is like $200. I also see that I can get a new motor for the condenser fan (part # 0-130-007-081-M14) which is much cheapter, about $76. This looks identical to the one on my fan assembly. My question is are they interchangeable? If I can remove my original, would this one fit in its place and perform the same?

Thank you very much for your help, in advance Paul

Thx,

Wayne

[cmonref]

Your full-on-all-the-****-time heat might be caused by an open circuit at the sensor in the windshield frame above the rear view mirror (if you have one there.) If so, remove the sensor and make sure it is making contact with the plug-on wire behind it. If the plug is loose giving an open circuit, the autoheat goes full-on.

Happens to me occasionally on the freeway in July at 98 degrees with the A/C running -- the heat comes on to the max. Pull over, re-make the connection, motor on happily and cooly.

[Skip1]

Paul,

First question, is it auto heat or manual levers? If auto heat, I can direct you to a long post which I initiated on a "system not working" issue. My problem was linked to BOTH the engine fan motor as well as the relay. There are also some tests you can do in order to test the 12 point harness (relay is a completely different matter). The rear engine blower on mine will initiate at all times when the heater is on, as well as if the engine starts heating up. If you're getting full heat in the cabin, that normally assumes that the flapper boxes between the heat exchangers and the floor ducting below the doors are closed, forcing the heat into the cabin. This can be due to either the servo not releasing, seized cables, or flapper box problems. I started what is now a rather long thread, but has excellent responses and tests (although it is based on the Auto-Heat, but the basics less the parts between the seats I believe are identical. You can PM me if you wish as well, and I'll help as best I can. I have an '84.

Auto Heat Query